To record high-resolution moving pictures on a single optical disc (storage medium), the optical disc needs to have an increased capacity for recording. An approach for achieving this is to provide plural writing-target layers (information writing layers) to an optical disc. Read-only single-sided double layer discs such as DVD-ROM, DVD-Video have been widely marketed, and are commercially available. Meanwhile, a standard for information-writable single-sided double layer optical discs has been already announced, and commercialization of such optical discs is just a matter of time.
Here, an optical-disc having plural writing-target layers has a problem of unnecessary reflected light component (stray light component) off a non-targeted layer which is a layer other than a writing-target layer to which information reading/writing is performed. Specifically, if a reflected light component off a writing-target layer (Hereinafter, targeted-layer) and a reflected light component off the non-targeted layer are detected, overlapping each other, an accurate measurement of a light quantity becomes difficult.
Technologies for this problem are disclosed for example in Patent document 1 (Japanese Patent No. 3372413 (Tokukaihei 9-0161282; Published on Jun. 20, 1997; Corresponding U.S. Pat. No. 5,881,035)) and Patent document 2 (Japanese Unexamined Patent Publication No. 319177 (Tokukai 2002-319177; Published on Oct. 31, 2002)).
Patent document 1 discloses a technique for restraining an influence of a stray light component which mixes in a focus signal. In the technique, only an influence of a stray light component is detected by using a configuration in which a light-receiving section for receiving a reflected light component off a targeted layer is provided with: a main light-receiving region for receiving a reflected light component off a targeted layer; and an auxiliary light-receiving region for receiving only a reflected light component off a non-targeted layer.
Further, similarly to Patent document 1, Patent document 2 discloses a technique for restraining crosstalk of a reproduced signal and a focus signal by providing an auxiliary light-receiving region for receiving a reflected light component (stray light component) off a non-targeted layer.
On the other hand, unlike a read-only (Play-only) disc, a writable disc has a problem of crosstalk to a tracking signal.
In view of this, a differential push-pull method (hereinafter DPP method) using three beams (one main beam and two sub beams) is generally employed as a tracking method. This DPP is a method in which an offset-free tracking signal is obtained by finding a differential amongst push-pull signals of the main beam and sub beams.
For example, in a case of using the above mentioned three beams for writing information in a DVD±R disc, the writing speed is proportional to the square of the light intensity of the main beam. On this account, the light intensity of the main beam needs to be increased to achieve a high-speed writing. In this case, the intensity ratio of the main beam versus each of the sub beams is set at for example 10:1 or 15:1, and as such, the light intensity of each sub beams is significantly low. This means that each detector (light-receiving section) for detecting the sub beam detects a light beam whose light intensity is extremely lower than the main beam.
Meanwhile, when writing or reading information into/from an optical disc in which plural writing-target layers are laminated, reflected light off a non-targeted layer (a layer not subject to information reading/writing) enters the light-receiving section as stray light including stray light components of the main beam and the sub beams. Since the stray light is light which is defocused by a layer interval from the writing-target layer, the stray light passes through a collector optics, and enters the light-receiving section without being focused. As such, the light-receiving section is irradiated with a light beam which is considerably larger than a light beam reflected off the layer to which information writing/reading is performed.
The stray light emitted in a wide range are incident on the light-receiving section used for tracking control, and causes an error in calculation of a tracking servo signal. This disturbs accurate tracking control.
The light density (a light quantity per a unit area) of such stray light is remarkably-low, when compared with a light quantity of the main beam or that of the sub beam. However, since the main beam has higher light intensity than the sub beams in the first place, a stray light component originated in the main beam gives a considerable amount of negative influence to the tracking control
In a case of recording information on a DVD±R by using the three beams, the use of technologies disclosed in Patent documents 1 and 2 for restraining stray light-caused error necessitates provision of the auxiliary light-receiving region which receives only reflected light off the non-targeted layer to each one of the light-receiving sections for respectively detecting tracking-use beams of the three beams. However, the provision of the auxiliary light-receiving region to each of the light-receiving sections causes an increase in the area of the entire light-receiving section, and complication of the associated circuits.
The stray light itself does not cause a large influence when it is incident on the light-receiving region for receiving the main beam. This is because the light density of the stray light is considerably low.
However, each of the light-receiving regions for receiving the sub beams receives a light beam whose light intensity (light density) is extremely lower than that of the main beam. Specifically, the light intensity of the sub beam is smaller than that of the main beam (e.g. 1/10 of the main beam), and the light-receiving region for receiving the sub beam receives a light beam having such a small light intensity. Thus, when the sub-beam light-receiving region receives the stray light (particularly main-beam-originated stray light component whose light intensity is stronger than the other stray light components), a significant influence is given to the light intensity of light received by the light-receiving section, even if the light intensity of the stray light component is smaller than that of light from a layer to which information reading/writing is performed. This consequently causes significant offset of DPP signals. Furthermore, an accurate tracking signal is not obtained, when the stray light component originated in the main beam having a higher light intensity than the sub beam enters the sub-beam light-receiving region.